1. Field of the Invention
This present invention relates to a marine seismic hydrophone array formed by a continous line array of piezoelectric film of continuous extent with discrete points of increased sensitivity formed over sections of relatively compressible substrate. The invention is useful to acoustic arrays in general and may be used in air, water and other acoustic mediums.
2. Summary of Related Art
Marine seismic hydrophones are typically formed in seismic cables as a collection of associated receiver elements. The marine seismic hydrophone cables are towed behind a marine seismic vessel or deployed from a seismic vessel to rest on the bottom of the ocean. The marine seismic hydrophone cables are used for performing ocean borne and ocean bottom seismic surveys to determine the presence of hydrocarbon bearing formations beneath the surface of the ocean floor. Seismic surveyors utilize acoustic energy sources such as air guns to generate acoustic energy pulses that penetrate and acoustically probe subsurface geological formations. The seismic surveyor hydrophone cables include hydrophones and/or geophones for detecting acoustic energy generated by the seismic sources and reflected from subsurface formations. The characteristics of these acoustic pulse reflections enable seismic surveyors to determine the nature of the formations from which the reflections emanate.
The marine seismic survey cables typically comprise encapsulated hydrophone and/or geophone sub-arrays, comprising electrical conductors, fiber optic conductors for digital telemetry, seismic sensor wiring, and cable/hydrophone buoyancy materials. For towable seismic streamers, internal buoyancy fill materials are provided to increase buoyancy and overcome the relatively heavy internal components of the hydrophone cables. The hydrophone cable buoyancy material enable the seismic streamer cables to become neutrally or slightly negatively buoyant in water. Neutral buoyancy or slightly negative buoyancy, as desired, enables easier streaming through the water and/or deployment of the seismic hydrophone cable to the ocean bottom. Typically, a flexible water proof jacket surrounds the seismic hydrophone cables to exclude water from the interior components of the cable and reduce frictional drag to the cables as they are streamed through the water or deployed to the ocean bottom.
Towed seismic streamer hydrophone cables range up to twelve kilometers long. The seismic hydrophone cables are typically towed beneath the water surface to avoid the acoustic and mechanical noise produced by surface wave action and other such seismically detrimental environmental factors. The streamer elevation or depth beneath the surface of the water is selected for the associated water surface conditions, the water depth, and for the desired seismic data frequency content. Positional control over streamer elevation is critical to data quality, data frequency content and to the reflected seismic signals received by hydrophones which are incorporated into the seismic streamers.
Conventional seismic streamers are typically fluid filled. Seismic streamers and ocean bottom cables typically contain acoustic detectors such as piezoelectric crystal hydrophones. Each seismic streamer or ocean bottom cable is circular in cross section. The seismic acoustic signals are typically recorded from groups of point sensors connected together to form a subarray. Each sub-array typically comprises a plurality (e.g. fourteen) individual sensors, and the sub-array centers of sensors are typically spaced at 12.5 meter intervals along the cable. The hydrophone sensors are electrically grouped into sub-arrays to enhance the desired signal and to reduce undesirable noise.
Conventional hydrophones have used ceramic buttons as acoustic sensing elements. More recently piezoelectric films has been employed as the acoustic sensing element in seismic hydrophone cables. Piezoelectric film produces an electrical signal when stressed or strained under an impinging acoustic sound wave or other force, such as towing tension and compression. The sensitivity of the piezoelectric film material is anisotropic so that the magnitude of the response varies with the direction of the applied acoustic stress. For instance, the piezo-stress constant, g for electrical measurements made across the thickness of a PVDF piezoelectric film varies by about 50% depending on whether the stress is applied in the thickness (i.e. xe2x80x9c3xe2x80x9d direction) or length direction (i.e. xe2x80x9c1xe2x80x9d direction). For PVDF film, g31 and g33 are 216xc3x9710xe2x88x923 Vm/N and xe2x88x92330xc3x9710xe2x88x923 Vm/N respectively. The relative differences in piezo-stress constant for other materials can be much greater. For copolymer film the ratio of g31/g33 is about 0.25. (Measurement Specialties Incorporated Technical Manual for Piezo-film Sensors, internet version updated August, 1998) The relative difference in piezo-stress constants of a film is further amplified by the way in which film is exposed to stress. Compared to a rigid-backed area of film exposed to pressure, the same film area exposed to the same pressure but backed by a compliant material may produce a signal more than 100 times as great. The rigid backed film produces a signal proportional to the thickness of the film. A compliant backed film produces a signal proportional to the span of film stretched. For PDVF, the piezo-stress constant for stress applied in the thickness direction is larger than that of the stretch direction but the length of material stretched is many times greater than the thickness and so the film is much more sensitive to stretch than to thinning. If a line array of rigidly backed piezo-film had small areas where the backing was complient, the film stretched over those areas would have enhanced sensitivity to pressure.
U.S. Pat. No. 5,774,423, U.S. Pat. No. 5,982,708, U.S. Pat. No. 5,883,857 and U.S. Pat. No. 6,108,274 disclose a piezoelectric acoustic sensor having one or more segments that are electrically coupled to provide a response corresponding to an acoustic pressure applied to the segments. Another piezoelectric film acoustic sensor is described in U.S. Pat. No. 5,361,240 wherein a piezoelectric film is wrapped around a mandrel. A hollow space or void is formed between the piezoelectric film and the mandrel provides pressure compensation to permit activation of the film. Another piezoelectric film hydrophone was described in U.S. Pat. No. 5,774,423 where a flexible piezoelectric film was encapsulated with a segmented housing. Two or more clam-shell type housings were fastened to a cable to form a hydrophone. A hollow space was provided to permit flexure of the piezoelectric material, and multiple hydrophones assembled to form a seismic array.
Thus, there are no known piezoelectric hydrophones that provide for the combination of a discrete array component sensitivity in a continuous line array. Thus there is a need for a piezoelectric hydrophone that provides for the combination of discrete array component sensitivity in a continuous line array.
This present invention provides a hydrophone array formed by a continuous line array of piezoelectric film with discrete points of increased sensitivity. The configuration provided by the present invention simultaneously provides the advantages and attributes of both the continuous line array and the multi-element discrete array. The line array and the multi-element array are designed to enhance or cancel specific frequency bands of signal noise and to enhance beam forming of the array. The piezoelectric hydrophone array can be extended and shaped into two-dimensional and three-dimensional hydrophone arrays.
The present invention comprises a continuous line array formed by a single piece of piezoelectric film with one or more points of enhanced sensitivity to alter the beam pattern or spectral sensitivity of the array. The invention is useful to acoustic arrays in general and may be used in air, water and other acoustic mediums. The electrical output of the entire array may be observed with one set of connectors, one positive and one negative lead. The effect of changing the beam pattern and/or spectral sensitivity applies to two and three-dimensional array, for example, multi-armed star, circular planar array and cylindrical array.